The present invention relates to optical detectors, and, in particular, to an optical detector for use in activating lights upon detection of infrared radiation from a person entering a room.
Many types of devices have been designed to detect the presence of a person in a room in order to automatically light the room when the person is in the room, and turn off the lights when the person leaves. One type of device detects the motion of a person by sending out and receiving back radar-type signals. Another type of device detects the infrared radiation emitted from a person's body. There are commercially available optical detectors for detecting infrared radiation. One type is a passive infrared (PIR) cell.
Such detectors have a relatively narrow angle of receiver sensitivity, or field of view. There is a maximum sensitivity when the infrared radiation is received straight-on, with the sensitivity decreasing as the angle from which the radiation is received increases. This has the effect of limiting the amount of a room which can be covered by such a device, requiring that it be strategically placed or that multiple detectors be used.
One solution to this problem is disclosed in U.S. Pat. No. 4,644,147. A pair of curved mirrors is arranged on either side of the PIR cell to intercept infrared light which has gone past the detector and redirect it back to the detector at an angle which is within the field of view of the detector.